Well, one thing that throws me a little is something I'm not sure has a solution. For instance, if you're using the collapsed-menu flyouts, they have the section header with the gray background--which sets the flyout off the background better than straight white with a shadow. But the problem is that with the default menu flyouts, section headers are completely moot, because they're already there. You need a way to accomplish the same effect without duplicating the flyout headers.

I also detected a bit of a delay between flyouts that was noticeable but so slight as to not make much of a difference.

See 18382.fix-glitch.diff. Firefox doesn't seem to like have two mouseout/mouseover events caused by always calling fold. If my comment does not reflect the intention of always calling fold then ignore the patch :)

I'm wondering if there should be more contrast between the flyout and what's below it. Perhaps adding a light gray background to the flyout or fading #wpbody when you're hovering over #admin menu would make sense.

The way the fold/unfold (used to) work is only in css depending on the class 'folded' on the body. This is broken now. Also when no js, the submenus are completely hidden with no way to reach them.

@duck_'s patch works but I don't see the point in having two handlers that do exactly the same thing in the first place. The fly-out 'headers' can be shown/hidden with css depending on the menu being folded or not. In short, this needs more work :)

This may be a candidate for another ticket another time, but how will the (new flyouts) menu be treated when the dashboard is resized with this new responsive admin? Will the hover still exist but just be moot in terms of touch screens? Will the menus expand down automatically at a certain screen width? Or maybe a tap to open animation without a page refresh? My motivation for asking is this: If the purpose of the flyouts was to make any menu accessible via one-click access, how will you match that for devices that don't innately support hover operations?

Sorry for late comment, but why does the flyout rely on a Javascript toggle class.

The experience is much richer by using the css :hover class to show the sub menus.
Try moving your mouse over several menus, you'll sometimes catch them overlapping
or being sluggish. Try the same with Javascript disabled, which is using just the CSS :hover, and
the result is much nicer.